Vertical FET with Sharp Junctions

ABSTRACT

VFET devices and techniques for formation thereof having well-defined, sharp source/drain-to-channel junctions are provided. In one aspect, a method of forming a VFET device includes: forming a SiGe layer on a substrate, wherein the SiGe layer as formed on the substrate is undoped; forming an Si layer on the SiGe layer, wherein the Si layer as formed on the SiGe layer is undoped; patterning fins in the Si layer; forming sacrificial spacers along sidewalls of the fins; forming recesses in the SiGe layer between the fins; growing an epitaxial material in the recesses, wherein the epitaxial material grown in the recesses includes a source and drain dopant; annealing the epitaxial material to diffuse the source drain dopant into the SiGe layer under the fins forming bottom source and drains of the VFET device; and removing the sacrificial spacers. A VFET device formed by the method is also provided.

FIELD OF THE INVENTION

The present invention relates to vertical field-effect transistor (VFET)devices, and more particularly, to VFET devices and techniques forformation thereof having well-defined, sharp source/drain-to-channeljunctions.

BACKGROUND OF THE INVENTION

As opposed to planar complementary metal oxide semiconductor (CMOS)devices, vertical field effect transistors (VFETs) are oriented with avertical fin channel disposed on a bottom source/drain and a topsource/drain disposed on the fin channel. VFETs are being explored as aviable device option for continued CMOS scaling beyond the 7 nanometer(nm) technology node.

A typical VFET device includes a vertical fin that extends upward fromthe substrate. The fin forms the channel region of the transistor. Asource region and a drain region are situated in electrical contact withthe top and bottom ends of the channel region, while the gate isdisposed on one or more of the fin sidewalls. One challenge forfabricating VFET is to achieve a sharp junction and low extensionresistance at the interface between the source/drain and the channel.One approach to form the bottom source/drain in a VFET process flow isvia ion implantation. However, implantation of dopant species tends todamage the vertical fin channel.

Another approach to forming a VFET bottom source/drain is bythermally-driven diffusion. While thermally-driven diffusion of dopantspecies can avoid damaging the fin channel, the diffusion process can bedifficult to control to achieve the desired sharp, well-definedjunction.

Therefore, techniques are needed for forming a VFET device with sharp,well-defined junctions.

SUMMARY OF THE INVENTION

The present invention provides vertical field-effect transistor (VFET)devices and techniques for formation thereof having well-defined, sharpsource/drain-to-channel junctions. In one aspect of the invention, amethod of forming a VFET device is provided. The method includes:forming a silicon germanium (SiGe) layer on a substrate, wherein theSiGe layer as formed on the substrate is undoped; forming a silicon (Si)layer on the SiGe layer, wherein the Si layer as formed on the SiGelayer is undoped; patterning fins in the Si layer; forming sacrificialspacers along sidewalls of the fins; forming recesses in the SiGe layerbetween the fins; growing an epitaxial material in the recesses, whereinthe epitaxial material grown in the recesses includes a source and draindopant; annealing the epitaxial material to diffuse the source draindopant into the SiGe layer under the fins forming bottom source anddrains of the VFET device; and removing the sacrificial spacers.

In another aspect of the invention, a VFET device is provided. The VFETdevice includes: a substrate; a SiGe layer disposed on the substrate;fins disposed on the SiGe layer, wherein the fins include undoped Si;recesses in the SiGe layer between the fins; and an epitaxial materialin the recesses that includes a source and drain dopant, wherein theSiGe layer under the fins also includes the source and drain dopant andforms bottom source and drains of the VFET device.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating thesource/drain-to-channel junction in a vertical field-effect transistor(VFET) device architecture according to an embodiment of the presentinvention;

FIG. 2 is a diagram illustrating dopant concentration from the undopedfin channel to the doped bottom source/drain according to an embodimentof the present invention;

FIG. 3 is a diagram illustrating a starting structure for forming a VFETdevice that includes a substrate, an undoped SiGe layer having beenformed on the substrate, and an undoped Si layer having been formed onthe undoped SiGe layer according to an embodiment of the presentinvention;

FIG. 4 is a diagram illustrating a sidewall image transfer (SIT) processwhereby at least one mandrel is formed on the undoped Si layer, andsidewall spacers are formed on opposite sides of the mandrels accordingto an embodiment of the present invention;

FIG. 5 is a diagram illustrating the mandrels having been removedselective to the sidewall spacers according to an embodiment of thepresent invention;

FIG. 6 is a diagram illustrating the sidewall spacers being used as finhardmask to pattern the undoped Si layer into one or more individualfins according to an embodiment of the present invention;

FIG. 7 is a diagram illustrating sacrificial spacers having been formedon opposite sides of each of the fins along sidewalls of the finsaccording to an embodiment of the present invention;

FIG. 8 is a diagram illustrating recesses having been formed in the SiGelayer (and a portion of the substrate) in between the fins according toan embodiment of the present invention;

FIG. 9 is a diagram illustrating a heavily-doped source/drain epitaxialmaterial having been grown in the recesses according to an embodiment ofthe present invention;

FIG. 10 is a diagram illustrating an anneal having been used to diffusea source/drain dopant from the source/drain epitaxial material into theSiGe layer under the fins according to an embodiment of the presentinvention;

FIG. 11 is a diagram illustrating the effective diffusivities ofphosphorous (P) in both Si and SiGe as compared to other dopants such asarsenic (As) according to an embodiment of the present invention;

FIG. 12 is a diagram illustrating the sacrificial spacers having beenremoved according to an embodiment of the present invention;

FIG. 13 is a diagram illustrating a bottom spacer having been formed onthe bottom source/drain according to an embodiment of the presentinvention;

FIG. 14 is a diagram illustrating a gate dielectric having beendeposited onto the fins and the bottom spacers, followed by a gateconductor according to an embodiment of the present invention;

FIG. 15 is a diagram illustrating an organic planarizing layer (OPL)having been deposited onto the gate conductor over the fins according toan embodiment of the present invention;

FIG. 16 is a diagram illustrating the OPL having been recessed below thefin hardmasks and below the tops of the fins according to an embodimentof the present invention;

FIG. 17 is a diagram illustrating the gate dielectric and gate conductorhaving been recessed according to an embodiment of the presentinvention;

FIG. 18 is a diagram illustrating a spacer material having beendeposited onto the fin hardmasks and recessed OPL according to anembodiment of the present invention;

FIG. 19 is a diagram illustrating the fin hardmasks and excess spacermaterial having been removed forming top spacers in between the tops ofthe fins according to an embodiment of the present invention;

FIG. 20 is a diagram illustrating top source/drains having been formedon the tops of the fins according to an embodiment of the presentinvention;

FIG. 21 is a diagram illustrating implementation of dopants of anopposite polarity to instead form a p-channel VFET device according toan embodiment of the present invention; and

FIG. 22 is a diagram illustrating the Y-vertical doping profile of thepresent device structure according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques for forming vertical field-effecttransistor (FET) devices having well-defined source/drain-to-channeljunctions. Of particular focus is the process for forming the bottomsource/drain below the vertical channel. Referring to FIG. 1, forexample, the present VFET design has a silicon germanium (SiGe) bottomsource drain (S/D) and an (undoped) silicon (Si) vertical fin channeldisposed on the bottom source/drain. A goal of the present techniques isto make the junction between the Si channel and the SiGe bottomsource/drain as sharp and well-defined as possible.

One parameter for assessing the characteristics of the sharpness of thejunction is junction width. See, for example, FIG. 2. FIG. 2 is adiagram illustrating dopant concentration as one moves from the undopedfin channel to the doped bottom source/drain. The region of the plotbetween the minimum/undoped fin channel and maximum/doped bottomsource/drain is the junction width. Ideally, the junction width is madeas small as possible which can be achieved by having a sharp interfacebetween the doped source/drain and the undoped channel. Withconventional techniques however, diffusion of dopant species into thesource/drain is difficult to control resulting in a larger junctionwidth due to insufficient diffusion of the dopant species throughout thesource/drain or the diffusion of the dopant species into the verticalchannel.

As will be described in detail below, the present techniques involveforming the bottom source/drain at the beginning of the process (priorto forming the gate or top source/drain). Namely, following patterningof the fins that will serve as the vertical channels of the device,sacrificial spacers are then used to cover/protect the vertical finchannel while heavily doped epitaxial silicon (Si) is grown in recessesin the bottom source/drain between the fins. This doped epitaxial Si isused to dope the bottom source/drain through drive in diffusion. Due toa faster diffusion rate of dopants through the source/drain (as comparedto the Si fin channel), sharp, well-defined junctions are produced.

An exemplary embodiment of the present techniques is now described forachieving the above-described sharp and well-definedsource/drain-to-channel junctions is now described by way of referenceto FIGS. 3-21. As shown in FIG. 3, the process begins with a substrate302, an undoped SiGe layer 304 being formed on the substrate 302, and anundoped Si layer 306 being formed on the undoped SiGe layer 304.

A variety of different substrate 302 configurations can be implementedin accordance with the present techniques. For instance, according toone exemplary embodiment, the starting substrate 302 is a bulksemiconductor wafer, such as a bulk Si, bulk Ge and/or bulk SiGe wafer.Alternatively, the substrate 302 can be a semiconductor-on-insulator(SOI) wafer. In general, a SOI wafer includes a SOI layer separated froman underlying substrate by a buried insulator.

When the buried insulator is an oxide it is often referred to as aburied oxide or BOX. The SOI layer can include any suitablesemiconductor, such as Si, Ge and/or SiGe.

According to an exemplary embodiment, an epitaxial process is used togrow both the undoped SiGe layer 304 on the substrate 302 and theundoped Si layer 306 on the undoped SiGe layer 304. Accordingly, growthof the (epitaxial) SiGe and Si layers will be templated off of theunderlying substrate 302. By way of example only, the undoped SiGe layer304 is formed on the substrate 302 to a thickness of from about 3nanometers (nm) to about 30 nm, and ranges therebetween, and the undopedSi layer 306 is formed on the undoped SiGe layer 304 to a thickness offrom about 10 nm to about 50 nm, and ranges therebetween.

As will become apparent from the description that follows, the undopedSi layer 306 will be used to form the vertical fin channels, and theundoped SiGe layer 304 will be used to form the bottom source/drain. Itis only after the bottom source/drain is formed that the gate and topsource/drain will be formed.

Undoped Si layer 306 is then patterned to form at least one vertical finchannel of the VFET device. Standard lithography and etching techniquescan be implemented to directly pattern the fins from a patterned finhardmask. Other patterning techniques are also contemplated herein. Forinstance, a sidewall image transfer (SIT) technique is shown illustratedin the figures. An advantage to an SIT process is that SIT permits thepatterning of feature sizes below that which can be achieved usingdirect patterning.

For instance, as shown in FIG. 4 SIT involves using standard lithographyand etching techniques to first pattern at least one mandrel 402 on theundoped Si layer. The mandrels 402 are a sacrificial structure usedmerely to place sidewall spacers for fin patterning. Thus, the mandrels402 are formed from a material that can be selectively removed relativeto the spacers such as amorphous silicon, poly-silicon, amorphouscarbon, silicon germanium (SiGe), an organic planarization layer (OPL),silicon dioxide (SiO₂) and/or silicon nitride (SiN).

Following patterning of the mandrels 402, sidewall spacers 404 areformed on opposite sides of the mandrels. See FIG. 4. By way of exampleonly, a suitable spacer material is blanket deposited onto and coveringthe mandrels and then patterned into individual spacers 404. Suitablematerials for spacers 404 include, but are not limited to, SiO₂, SiN,silicon oxynitride (SiON), silicon carbide (SiC) and/or amorphouscarbon. However, as provided above, the mandrels 402 will be removedselective to the spacers 404. Thus, the choice of mandrel and spacermaterial should be made to permit this selectivity. For instance, whenthe mandrels 402 are formed from an oxide such as SiO₂, the spacers 404can be formed from a nitride (SiN), or vice versa. As such, an oxide- ornitride-selective etch can be used to remove one (oxide or nitride)relative to the other. According to a non-limiting exemplary embodiment,the mandrels 402 are formed from an oxide material and the spacers 404are formed from a nitride material. Thus, after removal of the mandrels402 a nitride fin hardmask formed by the spacers 404 remains for finpatterning.

Namely, as shown in FIG. 5 the mandrels 402 have been selectivelyremoved. What remains are individual fin hardmasks formed by the spacer404 on the undoped Si layer 306. It is notable that the SIT processdescribed herein is a pitch doubling technique. Namely, there are nowtwo spacers 404/fin hardmask for each mandrel 402 originally patternedon the undoped Si layer 306.

As shown in FIG. 6, the spacers 404/fin hardmask are then used topattern the undoped Si layer 306 into one or more individual fins 602.As shown in FIG. 6, the etch used to pattern the fins 602 isnon-selective in the sense that it continues down through the undoped Silayer 306 and into a portion of the undoped SiGe layer 304. Thus, thefins 602 include a portion of the undoped SiGe layer 304 (i.e., the fins602 extend completely through the undoped Si layer 306 and part waythrough the undoped SiGe layer 304).

The next task is to grow heavily doped epitaxial Si in between the fins602 for source drain doping. However in order to protect the fins 602during this process, sacrificial spacers 702 are first formed onopposite sides of each of the fins 602 along the sidewalls of the fins602. See FIG. 7. Suitable materials for the sacrificial spacers 702include but are not limited to nitride spacer materials such as SiNand/or silicon nitride carbide. As provided above, the spacers 702 canbe formed by blanket depositing the spacer material onto the fins 602and then patterning the spacer material into the individual sacrificialspacers 702. By ‘sacrificial’ it is meant that the spacers 702 will beremoved later on in the process after the bottom source/drain has beenformed.

As shown in FIG. 8, the bottom source/drain is next recessed between thefins. In the present VFET architecture, the fins 602 will form thechannel regions of the VFET and the SiGe layer 304 beneath the fins(which will become doped later in the process) will used to form thebottom source/drain. According to an exemplary embodiment, an isotropicetching process such as a wet etch is used to form recesses 802 betweenthe fins 602. During this recess etch, the fins 602 are protected by thesacrificial spacers 702 along their sidewalls and the spacers 404/finhardmask at their tops. Thus, the recess etch is limited to thesource/drain in between the fins 602.

100451 As shown in FIG. 8, the recesses 802 extend through the undopedSiGe layer 304 and partway into the substrate 302. A heavily dopedsource/drain epitaxial material will be grown in the recesses 802 forsource/drain doping. This epitaxial growth will be templated off of the(now-exposed) substrate 302 within the recesses 802.

Namely, as shown in FIG. 9 a source/drain epitaxial material 902 isgrown in the recesses 802. For source/drain doping, the epitaxialmaterial 902 is heavily doped with an n-type or a p-type source/draindopant, e.g., at a concentration of from about 4×10²⁰ atoms/cm³ to about2×10²¹ atoms/cm³, and ranges therebetween. By way of example only, theepitaxial material 902 can be doped in-situ while being grown in therecesses 802. Alternatively, dopant implantation techniques can beemployed following growth of the epitaxial material 902 in the recesses802. According to an exemplary embodiment, the epitaxial material 902 inthe recesses 802 is phosphorous (P)-doped epitaxial Si (Si:P). This willresult in the formation of an n-channel VFET device by the presentprocess. In another exemplary embodiment, an epitaxial silicon dopedwith both carbon and phosphorous (Si:C(P)) is grown in the recesses 802.The Si:C(P) preferably has an atomic carbon concentration of from about0.2% to about 3% and ranges therebetween, such as an atomic carbonconcentration of from about 0.5% to about 1.5% and ranges therebetween.It is to be understood that the total amount of carbon in a crystallinesemiconductor layer may be higher than the substitutional amount. Thistoo will result in the formation of an n-channel VFET device by thepresent process. However, embodiments are described below where a dopantof the opposite polarity is employed to form a p-channel VFET.

A thermally-driven diffusion of the source/drain dopant (e.g.,phosphorous) from the source/drain epitaxial material 902 is then usedto form source/drain extensions 1002 by diffusing the source/draindopant into the SiGe layer 304 under the fins 602. See FIG. 10.According to an exemplary embodiment, the thermal diffusion is performedby annealing the source/drain epitaxial material 902 at a temperature offrom about 900° C. to about 1200° C. and ranges therebetween, for aduration of from about 1 minute to about 10 minutes and rangestherebetween.

Advantageously, the faster diffusion rate of the source/drain dopant(e.g., phosphorous) in the source/drain SiGe layer 304 as compared to inthe Si of the fins 602 is leveraged to establish a sharp junction underthe fins 602. See FIG. 10. Specifically, since the diffusion rate of thesource/drain dopant (phosphorous in this example) is faster in thesource/drain SiGe than in the fin Si, for an anneal of a given durationthe dopant will diffuse rapidly throughout the source/drain with littleto no diffusion into the fin. The result is a sharp interface betweenthe (doped) source/drain and the (undoped) fin channel.

By way of example only, the effective diffusivities of arsenic (As) andphosphorous (P) in both Si and SiGe under equilibrium conditions isshown illustrated in FIG. 11 (see Eguchi et al. “Comparison of arsenicand phosphorus diffusion behavior in silicon-germanium alloys,” AppliedPhysics Letters, vol. 80, no. 10, pgs. 1743-1745 (March 2002), thecontents of which are incorporated by reference as if fully set forthherein). In FIG. 11, circles are used to identify the plotscorresponding to the diffusivity of arsenic (As) in both Si and SiGe andthose corresponding to the diffusivity of phosphorous (P) in both Si andSiGe. Both dopants (As and P) show a higher diffusivity in SiGe (ascompared to Si) which increases linearly with an increase in annealingtemperature. However, phosphorous exhibits an overall higher diffusivityin SiGe thus making it an ideal dopant for the present techniques.

Now that the bottom source/drains have been formed, the process tocomplete the VFET device involves forming gates alongside the fins 602and source/drains on top of the fins 602. To enable these furtherprocessing steps, the sacrificial spacers 702 are now removed. See FIG.12. As provided above, the sacrificial spacers 702 can be formed from asuitable nitride spacer material, and as such a nitride-selective etchcan be employed for their removal. Removal of the sacrificial spacers702 exposes the sidewalls of the fins 602.

A bottom spacer 1302 is then formed on the bottom source/drain. Bottomspacer 1302 will offset the gate (formed as described below) from thebottom source/drain. A counterpart top spacer will too be formed thatseparates the gate from the top source/drain. See below. Suitablematerials for the bottom spacer 1302 include, but are not limited to,oxide spacer materials such as SiO₂ and/or nitride spacer materials suchas SiN.

According to an exemplary embodiment, the bottom spacers 1302 are formedusing a directional deposition process whereby the spacer material isdeposited onto the bottom source/drain and fins 602 with a greateramount of the material being deposited on the horizontal surfaces, ascompared to the vertical surfaces. To use an illustrative example, agreater thickness of the spacer material will be deposited on top of thesource/drain in between the fins 602 than along the sidewalls of thefins 602. Thus, when an etch is used on the spacer material, the timingof the etch needed to remove the spacer material from the verticalsurfaces will leave the bottom spacers 1302 shown in FIG. 13 on top ofthe source drain since a greater amount of the spacer material waspresent on the bottom source/drain to begin with. By way of exampleonly, a high density plasma (HDP) chemical vapor deposition (CVD) orphysical vapor deposition (PVD) process can be used for directional filmdeposition, and an oxide- or nitride-selective (depending on the spacermaterial) isotropic etch can be used to remove the (thinner) spacermaterial deposited onto the vertical surfaces.

To form the gates of the VFET device, a gate dielectric 1402 is firstdeposited onto the fins 602 and bottom spacers 1302, followed by a gateconductor 1404. See FIG. 14. In the exemplary embodiment illustrated inthe figures, both the gate dielectric 1402 and the gate conductor 1404are conformal layers. However, that is merely one example. For instance,the gate conductor can instead fully fill the space in between the fins.See, for example, U.S. Pat. No. 9,627,511 B1 issued to Cheng et al.,entitled “Vertical Transistor Having Uniform Bottom Spacers,” thecontents of which are incorporated by reference as if fully set forthherein.

According to an exemplary embodiment, a metal gate is formed wherein thegate conductor 1404 is a metal or combination of metals and the gatedielectric 1402 is a high-κ dielectric. For instance, the gate conductor1404 is a workfunction setting metal. The particular workfunction metalemployed can vary depending on whether an n-type or p-type transistor isdesired. Suitable n-type workfunction setting metals include, but arenot limited to, titanium nitride (TiN), tantalum nitride (TaN) and/oraluminum (Al)-containing alloys such as titanium aluminide (TiAl),titanium aluminum nitride (TiAlN), titanium aluminum carbide (TiAlC),tantalum aluminide (TaAl), tantalum aluminum nitride (TaAlN), and/ortantalum aluminum carbide (TaAlC). Suitable p-type workfunction settingmetals include, but are not limited to, TiN, TaN, and tungsten (W). TiNand TaN are relatively thick (e.g., greater than about 2 nm) when usedas p-type workfunction metals. However, very thin TiN or TaN layers(e.g., less than about 2 nm) may also be used beneath Al-containingalloys in n-type workfunction stacks to improve electrical propertiessuch as gate leakage currents. Thus, there is some overlap in theexemplary n- and p-type workfunction metals given above.

The term “high-κ” as used herein refers to a material having a relativedielectric constant κ which is much higher than that of silicon dioxide(e.g., a dielectric constant κ=25 for HfO₂ rather than 4 for silicondioxide). Suitable high-κ gate dielectrics include, but are not limitedto, hafnium oxide (HfO₂) and/or lanthanum oxide (La₂O₃).

As shown in FIG. 15, a dielectric such as an organic planarizing layer(OPL) 1502 is next deposited onto the gate conductor 1404 over the fins602. In the particular example shown illustrated in the figures, thespaces in between the fins 602 left by the conformal gate dielectric1402/gate conductor 1404 are now filled in by the OPL 1502.

As deposited, the OPL 1502 surrounds and fully covers the fins 602. SeeFIG. 15. However, access to the tops of the fins 602 is needed in orderto form the top source/drains. As such, as shown in FIG. 16 the OPL 1502is then recessed to expose the tops of the fins 602. Since the objectiveis to expose the tops of the fins 602, it is notable that the OPL 1502needs to be recessed below the spacers 404/fin hardmask (which willsubsequently be removed) and below the tops of the fins 602 (i.e., a topof the recessed OPL is now below the tops of the fins). See FIG. 16.Following recess of the OPL 1502, the gate dielectric 1402 and gateconductor 1404 are also recessed. See FIG. 17. As shown in FIG. 17,recessing the gate dielectric 1402 and gate conductor 1404 fully exposesthe spacers 404/fin hardmask at the tops of the fins 602.

As highlighted above, a counterpart top spacer is needed to offset thegate from the top source/drain. To form the top spacer, a spacermaterial 1802 is next deposited onto the spacers 404/fin hardmask and(recessed) OPL 1502. See FIG. 18. As above, suitable spacer materialsinclude, but are not limited to, oxide spacer materials such as SiO₂and/or nitride spacer materials such as SiN. In the same manner asdescribed above, a directional deposition process (such as HDP CVD orPVD) can be employed which, as shown in FIG. 18, results in thedeposited spacer material 1802 being thicker on the horizontal surfaces(e.g., T1) as compared to on the vertical surfaces (e.g., T2, whereinT1>T2).

A planarizing etch such as chemical-mechanical polishing (CMP) is thenused to remove the spacers 404/fin hardmask and with it excess spacermaterial 1802. The result is top spacers 1902 being formed in betweenthe (now exposed) tops of the fins 602. See FIG. 19. Selective epitaxialgrowth can then be used to form top source/drains 2002 on the tops ofthe fins 602. See FIG. 20. As with the bottom source/drains, the topsource/drains 2002 can be doped with an n-type dopant. For instance, byway of example only, the top source/drains 2002 can be formed fromin-situ phosphorous-doped SiGe (SiGe:P). Any further processing, ifneeded, can be performed to complete the device such as forming contactsto the top source/drain and/or any subsequent metallization, etc.

In the above example, an n-type dopant (e.g., phosphorous) is used inthe top/bottom source/drain to form an n-channel VFET device. Asprovided above, however, a dopant of the opposite polarity can insteadbe employed to form a p-channel VFET. The process for forming thep-channel VFET would be the same as that described above, except with avariation in the dopant employed for the source/drain, i.e., a p-typerather than n-type dopant. Suitable p-type dopants include, but are notlimited to boron (B). Thus, for instance, as shown in FIG. 21 theepitaxial material 902 grown in the recesses 802 can instead be boron(B)-doped SiGe (SiGe:B). Likewise, the top source/drains 2002 can beformed from in-situ boron (B)-doped SiGe (SiGe:B). This will result inthe formation of a p-channel VFET device by the present process.

As provided above, the present techniques result in well-defined, sharpjunctions between the bottom source/drain and the vertical fin channel.See, for example, FIG. 22, which illustrates the Y-vertical dopingprofile of the device structure shown in FIG. 10. As shown in FIG. 22,the phosphorous (P) diffuses in the fin bottom (substrate and SiGe) fromthe doped Si:P epitaxy in the recesses. There is a higher dopingconcentration in the SiGe due to the faster P diffusion in the SiGe thanin the Si. According to an exemplary embodiment, the junction has adoping gradient of less than 5 nanometers (nm) per decade, meaning thatthe doping concentration drops 1 order of magnitude within a 5 nm range.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

1. A method of forming a vertical field-effect transistor (VFET) device,the method comprising the steps of: forming a silicon germanium (SiGe)layer on a substrate, wherein the SiGe layer as formed on the substrateis undoped; forming a silicon (Si) layer on the SiGe layer, wherein theSi layer as formed on the SiGe layer is undoped; patterning fins in theSi layer by forming fin hardmasks on the Si layer, and etching the finsin the Si layer using the fin hardmasks; forming sacrificial spacersalong sidewalls of the fins; forming recesses in the SiGe layer betweenthe fins; growing an epitaxial material in the recesses, wherein theepitaxial material grown in the recesses comprises a source and draindopant; annealing the epitaxial material to diffuse the source draindopant into the SiGe layer under the fins forming bottom source anddrains of the VFET device; removing the sacrificial spacers, forming abottom spacer on the bottom source and drains of the VFET device;depositing a gate dielectric onto the fins and the bottom spacer;depositing a gate conductor onto the gate dielectric; depositing anorganic planarizing layer (OPL) over the fins; recessing the OPL belowtops of the fins; recessing the gate dielectric and the gate conductorto expose the fin hardmasks on the tops of the fins; forming a topspacer on the OPL in between the tops of the fins; removing the finhardmasks; and forming top source and drains of the VFET device on thetops of the fins.
 2. The method of claim 1, wherein the substratecomprises a semiconductor selected from the group consisting of: Si, Ge,SiGe, and combinations thereof.
 3. The method of claim 1, wherein thefins extend partway through the SiGe layer.
 4. The method of claim 1,wherein the sacrificial spacers comprise a nitride spacer materialselected from the group consisting of: silicon nitride (SiN), siliconnitride carbide, and combinations thereof.
 5. The method of claim 1,wherein the recesses extend partway through the substrate.
 6. The methodof claim 1, wherein the source and drain dopant comprises phosphorous(P), and wherein the epitaxial material grown in the recesses isselected from the group consisting of: Si:P and Si:C(P).
 7. The methodof claim 1, wherein the source and drain dopant comprises boron (B), andwherein the epitaxial material grown in the recesses comprises SiGe:B.8. The method of claim 1, wherein the epitaxial material grown in therecesses comprises the source and drain dopant at a concentration offrom about 4×10²⁰ atoms/cm³ to about 2×10²¹ atoms/cm³ and rangestherebetween.
 9. The method of claim 1, wherein the annealing isperformed at a temperature of from about 900° C. to about 1200° C. andranges therebetween, for a duration of from about 1 minute to about 10minutes and ranges therebetween.
 10. (canceled)
 11. The method of claim1, wherein the fin hardmasks are formed on the Si layer using a sidewallimage transfer (SIT) process.
 12. (canceled)
 13. The method of claim 1,wherein the gate dielectric comprises a high-κ dielectric selected fromthe group consisting of: hafnium oxide (HfO₂), lanthanum oxide (La₂O₃),and combinations thereof.
 14. The method of claim 1, wherein the gateconductor comprises a workfunction setting metal selected from the groupconsisting of: titanium nitride (TiN), tantalum nitride (TaN), analuminum (Al)-containing alloy, titanium aluminide (TiAl), titaniumaluminum nitride (TiAlN), titanium aluminum carbide (TiAlC), tantalumaluminide (TaAl), tantalum aluminum nitride (TaAlN), tantalum aluminumcarbide (TaAlC), and combinations thereof.
 15. The method of claim 1,wherein the bottom spacer and the top spacer each comprises a materialselected from the group consisting of: silicon oxide (SiO₂), SiN, andcombinations thereof.
 16. The method of claim 1, wherein the top sourceand drains comprise SiGe:P.
 17. The method of claim 1, wherein the topsource and drains comprise SiGe:B. 18-20. (canceled)